This invention relates to the field of impact printing and typewriting with particular emphasis on single element typewriters where velocity control is desired for the purpose of improved quality of the final print.
In the design of typewriters and single element printers, many compromises are necessitated by conflicting mechanical or physical requirements. This is particularly true with respect to the single element impact printers in the area of impact intensity or impact force. For example, the IBM Selectric Typewriter has two impact intensity levels, a high level and low level. The high level impact intensity accommodates letters, numbers and other large area graphic symbols. The low level impact intensity is utilized with the punctuation marks to reduce the forcing of small area symbols, such as period, colon, semicolon, dash, underscore and comma, deeply into the paper causing embossing.
There has been found a general correlation between the area of the character to be printed and the impact force sufficient to obtain substantially complete ink transfer from a ribbon to the printed page. For high quality printing, it is necessary to accomplish substantially complete transfer of all the ink layer from a ribbon which comprises an ink layer and a film plastic substrate. The impact level must also be sufficient to insure the adhesion of the ink to the paper at the time of impact.
At the same time, the impact forces generated between the type element and the platen supporting the record sheet bring about noise. With an increased awareness on noise in the office environment, it is desirable to reduce noise to the minimum. It can be seen that the desire to reduce noise brings about a desire to reduce the impact level or force while at the same time, the desire for quality printing brings about a desire for a higher level force to insure uniform characters on the printed page. These inconsistent desires bring about a need for compromise. The prior art compromises have precisely defined impact levels of a minimal number, i.e. two (2) in the IBM Selectric Typewriter, or a slightly larger number of impact levels with more complex mechanisms for accomplishing the appropriate selection thereof, as found in the IBM Electronic Typewriters 50, 60 and 75 which have three (3) levels of impact intensity.
The more impact intensity levels designed into the typewriter, the more complex the mechanisms necessary to properly generate those impact intensity levels. The more complex machine is more costly and further contributes to mechanical noise. The other prior art approaches of only two intensity levels to prevent the embossing by extremely small characters results in problems such as uneven print color, excessively high printing noise, and inherent incompatability of impact requirements between characters in upper and lower case due to differences in size and incompatability of the character size and impact level where foreign languages are typed on the typewriter by change of the print element, side and overprinting, and incompatability of the impact intensity levels when the typewriter is changed from a ten to a twelve pitch mode of operation with the corresponding reduction in typeface area.
Uneven print color results from the fact that some characters such as the numeral one and lower case "l" have considerably less surface area than the surface area of other lower case characters such as the lower case m and the capital letters, particularly the large capital letters such as capital M and capital W. The color of the character printed will to some extent be a result of the amount of impact force and embossing which occurs at printing. The smaller characters, i.e. l, i, will tend to penetrate the paper deeper and there will be a broader stroke to that character than a similar width line in a larger area character such as M and W.
Printing noises generated by the impact of the type element against the print page and platen will, with very limited selection of impact levels, such as 2, result in a substantially higher impact level and noise level than that actually required for quality printing in lower case typing.
The use of multiple interchangeable print elements on single element typewriters provide for multiple language capability from a single typewriter. With the substitution of a foreign language head or element onto the typewriter, the pecularities of the alphabetic and graphic characters on that head will result, in some cases, in a very small area symbol such as the umlaut and grave being printed with high velocity and resulting in a high degree of embossing as well as excessive machine noise.
In some cases, typewriters have been provided with a manual intensity level control whereby the operator may override the print velocity mechanism in such a way as to provide an altered print velocity. Even though this manual means exists on typewriters, indications are that a high percentage of the time, the device is not utilized and, therefore, is of no substantial benefit to the user.
The prior art has attempted to minimize or overcome some of these shortcomings by several different methods. The technique of having two separate print intensities is accomplished in two ways in the prior art devices. One is illustrated in U.S. Pat. No. 3,239,049 to W. F. Voit, Jr., and illustrates the technique found in the IBM Selectric Typewriter referred to above. This technique utilizes keyboard coding to shift, through a mechanical connection, a member which, in turn, acts to shift the cam follower from a high velocity lobe to a low velocity lobe on the print cam or vice versa. This device is dependent upon keyboard coding and, therefore, exhibits some of the problems which are described above and, to some extent, limits the choices of foreign language print elements if compatability of the print face and impact level must be maintained. Otherwise, compromises are required to accommodate the keyboard coded impact levels dictated by the mechanical hardware.
U.S. Pat. No. 3,980,169 shows a impact control where a heavy spring provides the printing force by acting upon a drive member. The drive member is stopped or blocked from completing its otherwise normal flight path by a member which is selectively inserted into its path, thereby prematurely terminating the powered portion of the movement of said member, thereby imparting a smaller energy level to the rocker carrying the element.
Other techniques involve the position of the type element controlling the length to which a spring is stretched. The spring then provides a force for printing.